The invention relates to a screen for screening pulp, the screen comprising a body, a screen cylinder arranged inside the body and having a cylinder surface which is provided with openings and forms a screening surface of the screen for screening the pulp, a feed chamber and a feed connection connected with it for feeding the pulp to be screened to the screen, and an accept chamber and an accept connection connected with it for removing the accepted pulp fraction from the screen, the accept chamber being arranged between the screening surface of the screen cylinder and the screen body.
The invention also relates to a screen for screening pulp, the screen comprising a body, a screen cylinder arranged inside the body and having a cylinder surface which is provided with openings and forms a screening surface of the screen for screening the pulp, a feed chamber and a feed connection connected with it for feeding the pulp to be screened to the screen, and an accept chamber and an accept connection connected with it for removing the accepted pulp fraction from the screen, the feed chamber being arranged between the screening surface of the screen cylinder and the screen body.
The invention further relates to a method for screening pulp, wherein the pulp is screened by a screen comprising a body, a screen cylinder arranged inside the body and having a cylinder surface which is provided with openings and forms a screening surface of the screen for screening the pulp, a feed chamber and a feed connection connected with it for feeding the pulp to be screened to the screen, and an accept chamber and an accept connection connected with it for removing the accepted pulp fraction from the screen, the accept chamber being arranged between the screening surface of the screen cylinder and the screen body.
The invention further relates to a method for screening pulp, wherein the pulp is screened by a screen comprising a body, a screen cylinder arranged inside the body and having a cylinder surface which is provided with openings and forms a screening surface of the screen for screening the pulp, a feed chamber and a feed connection connected with it for feeding the pulp to be screened to the screen, and an accept chamber and an accept connection connected with it for removing the accepted pulp fraction from the screen, the feed chamber being arranged between the screening surface of the screen cylinder and the screen body.
Pulp is screened in a treatment phase before it is supplied to a paper or board machine or a similar apparatus to remove from the pulp various impurities, slivers and other objects deteriorating the quality of a paper or board web to be manufactured. The apparatus for screening pulp is called a screen. The screen comprises a pressure-proof frame, i.e. a body, inside of which there is a screening surface with openings for screening the pulp. The screen also comprises a feed chamber and a feed connection connected with it for feeding the pulp to be screened to the feed chamber inside the screen. The screen further comprises an accept chamber and an accept connection connected with it for removing the accepted pulp fraction, i.e. the accept, from the accept chamber out of the screen and for feeding it forward in the process, and a reject connection for removing the fraction rejected in the screen, i.e. the reject, from the screen for further processing. The screen volume inside the body is thus divided into the feed chamber in connection with the feed connection, the accept chamber in connection with the accept connection, and a reject space in connection with the reject aggregate. The screen also typically comprises a separate discharge connection, through which the impurities accumulated on the bottom of the accept chamber can be removed from the screen.
A commonly used screen type comprises a cylindrical body, inside of which there is a screen cylinder with a cylindrical screening surface with openings for screening pulp. The openings in the screening surface may be, for instance, round or longitudinal holes or parallel slots in the surface of the screen cylinder. The screen may also comprise a rotor arranged inside the cylindrical screening surface for circulating the pulp in the screen, and foil wings fastened to the frame of the rotor and causing a strong suction pressure pulse on the screening surface while the rotor rotates, due to which the reject pulp and the fibres accumulated on the screening surface are removed from the screening surface and returned into the pulp mixture. The screen provided with a cylindrical body and a cylindrical screening surface may be used in both vertical and horizontal positions.
In screens with a cylindrical body structure, the accept chamber may be restricted to the region between the screening surface and the screen body, and the feed chamber is typically arranged at the other end of the screen. There are also screens where the feed chamber may be restricted to the region between the screening surface and the screen body, and the accept chamber is located inside the screen cylinder and at the other end of the screen. Cylindrical screens, however, are often too narrow for a flow, which causes an uneven flow through the screening surface of the screen. In practice, there have appeared dirtiness problems so that a disadvantageous geometry of the accept chamber may cause, for instance, a kaolin slurry to precipitate on the bottom of the accept chamber. Further, it is difficult to empty the cylindrical screen completely during factory shutdowns, for instance. If the application field of the screen is a machine screen, in which a clean, screened pulp is supplied to the machine screen and further to the paper or board machine, the machine screen itself may cause problems, such as web breaks, if its shape is disadvantageous for the flow. This makes the surfaces dirty and causes fibre bundles to accumulate at dead flow points.
One solution for adjusting the flows through the screening surface would be to increase the diameter of the screen body. This, however, increases the manufacturing costs of the screen considerably, a significant part of which is constituted by the flange joints required. It is also more expensive to manufacture the accept opening and its reinforcements when pressure levels become higher, because welding and arranging of an accept aggregate is craftsmanship which requires great professional skills. Further, the rest of the frame structure of the screen may comprise many different structural principles, wherefore the automation of screen manufacture and welding in particular is very limited.
U.S. Pat. No. 5,318,186 discloses a screen including two successive screen baskets and a substantially cylindrical screen body having cone and cylinder parts and convex ends. In the longitudinal direction of the screen, the diameter of the body is larger in the middle of the screen than at its ends, and this solution has been selected on the basis of the space required by the support structure needed for supporting the two screen baskets in the screen. Such a body structure does not, however, prevent the formation of dead flow points, nor does it have an advantageous effect for improving the pulp flow through the screening surface. As to the use of material, it is also very disadvantageous to manufacture the cone structure disclosed in the publication from a sheet material.